Photocomposition of text material has evolved and developed along various avenues, but the most popular and successful forms of photocomposition machines present a rotary font of alphanumeric subject matter in the form of a disc with the characters to be printed represented by transparent areas near the peripheral edge of the disc.
A separate series of alternate transparent and opaque areas provides the source for information to control the logic of the photocomposer in locating a desired character and focuses the image thereof on a plane where a photosensitive sheet is located.
Some photocomposing machines are capable of composing in only a relatively minor variation of size. More sophisticated, later-generation machines are now available which will provide various degrees of enlargement from character transparencies on the same disc. Therefore, it is possible to compose widely varying size images intermixed.
The usual prior art practice is to run the disc continuously but at a speed corresponding to the acceptable speed suitable for projecting the largest character, which is slow in comparison to relatively smaller characters. Another approach is to use a stepper motor and bring each character font to a halt for the required exposure time. The larger characters require a slower speed because the movement of the disc is much more obvious in printed matter of large size. Perfect copy can only be obtained by a disc sitting in a stationary position, but the use of means to bring the disc to a halt slows the process unacceptably for normal commercial composition.